US President Donald Trump has expressed optimism about his upcoming meeting with North Korean leader Kim Jong Un, saying the one-time opportunity "will not be wasted."

Trump posted messages on Twitter on his way to Singapore, where he will meet Kim on Tuesday. 

Trump said the summit with Kim is "a chance to achieve a truly wonderful result for North Korea and the world."

He writes "it will certainly be an exciting day" and he knows that Kim "will work very hard to create peace and prosperity for his land".

Trump says that is "something that has rarely been done before."The president added he has a feeling that what he calls the one-time opportunity "will not be wasted."

Trump made the remark to reporters on Saturday in Canada, where he was attending the Group of 7 summit.Trump was upbeat about his meeting with Kim.

He said he believes the North Korean leader "is going to do something very positive for his people."

He added that he's "hopeful" the 2 leaders will "like each other."Trump and Kim are due to meet in Singapore on Tuesday.